The invention relates to a hydraulic power steering for motor vehicle having a mechanical steering mechanism connected to the steered vehicle wheels for steering the wheels with an element which has a normal position out of which it can be moved against the force of a spring depending on the forces effective between the steered vehicle wheels and the steering mechanism, a servo-valve arrangement controllable by the position of the element and a hydraulic servo-motor controllable by the servo valve and connected to the steered vehicle wheels or to the steering mechanism connected to the steered vehicle wheels for steering the vehicle wheels.
Such power steerings are generally known and are installed in motor vehicles in large numbers. The operating member is generally a driver operated steering wheel (or some other operating handle). The servo-motor is energized depending on the forces transmitted between the steering wheel and the steered vehicle wheels so as to reduce the manual force to be applied to the steering wheel.
In present power steerings, the servo-valve arrangement usually has a so-called open center that is pressurized hydraulic fluid passes through the servo-valve arrangement also when the steering is in a straight driving position for which only small forces are necessary at the steering wheel. Nevertheless, power is consumed in this position for maintaining the pressurized hydraulic fluid flow.
Power steerings with so-called closed centers with no pressurized fluid flow in a straight driving position and with only small manual forces wherein no power is needed are also known. In this case, the servo-valve arrangement opens only when a force is applied to the steering wheel whereby communication is established between the pressurized fluid source or, respectively, a pressurized fluid storage space and the serv-omotor. Such systems, however, are not in practical use. The reason is probably that the opening behavior of servo valve arrangements with closed centers is somewhat problematic as the servo forces are initiated suddenly that is not smoothly so that the transition is quite noticeable.
All power steering systems presently in use have a mechanical connection between the steering wheel and the steered vehicle wheels which provides for a mechanical coupling between the steering wheel and the steered vehicle wheels to insure greatest safety.
It is well known, however, to operate with the steering wheel or operating member only a setting means which cooperates, via a control system, with a control device whose control movements are adjusted by a position setting- to actual position comparison and are essentially proportional to the adjustment movements of the operating member. Such control arrangements utilizing electronic control systems are used, for example, in airplanes for the adjustment of the wing flaps, the elevators and the rudders. These control systems which are also called "fly-by-wire" systems are in the meantime so safe that they are used not only in military aircraft but also in commercial passenger airplanes.
It is the object of the present invention to utilize such a concept also with vehicle steering systems together with proven components of conventional power steering systems in such a way that the power requirements for the power steering are substantially reduced.